The present invention relates to fused evanescent wave couplers and, in particular, to a single-mode fiber optic multiplexer/demultiplexer.
Wavelength division multiplexing has become an important tool for increasing the data transmission capacity of fiber optic communication systems and local area networks. Multiplexer/demultiplexers have been fabricated in multimode fiber telemetry systems using prisms, gratings and interference filters to combine or separate two wavelengths. Because of the lower loss and higher bandwidth of single-mode fiber telemetry systems, there is increasing interest in developing practical single-mode fiber optic multiplexer/demultiplexers for these systems.
The prior art has suggested that high quality single-mode fiber optic multiplexer/demultiplexers could be fabricated by using the natural wavelength dependence of the coupling in evanescent wave couplers. For an evanescent wave coupler to serve as a multiplexer/demultiplexer at two preselected wavelengths, however, it is necessary to adjust the wavelength dependence of the coupling ratio (defined as the coupled power divided by the sum of the transmitted and coupled power) until the coupling ratio is equal to 05 at one wavelength and 100% at the other wavelength. Polished substrate couplers have been used to fabricate such multiplexer/demultiplexers with the wavelength characteristics of the devices adjusted by changing the relative position between the two polished substrates. In principle, one should be able to obtain multiplexer/demultiplexers at any two wavelengths of interest by appropriately adjusting the wavelength dependence of the coupling ratio for these polished substrate couplers. However, in general, the long term stability and environmental sensitivity of these polished couplers has been shown to be inferior to that of fused couplers and thus there has been significant interest in developing fused coupler multiplexer/demultiplexers.
The technology required to produce high quality fused evanescent wave couplers has progressed to the point that fiber optic couplers with less than 0.05 dB loss and virtually no temperature or polarization sensitivity are now readily available. Previous efforts to develop fused coupler wavelength multiplexer/demultiplexers, however, have been limited to measuring the wavelength dependence of these couplers and mapping out wavelengths where the coupler would function as a multiplexer.
The present invention is an improvement over the prior art and features a process for fabricating a multiplexer/demultiplexer at two preselected wavelengths. The present invention also is a novel fused coupler where the wavelength dependence of the coupling ratio has been altered such that the coupler acts as a wavelength multiplexer/demultiplexer at wavelengths of 1.32 .mu. and 1.55 .mu.. These wavelengths were selected since laser-transmitters at 1.32 .mu. and 1.55 .mu. are becoming the light sources of choice for many high speed single mode telemetry systems because of the low loss and low dispersion exhibited by fibers at these wavelengths.